1. Field of the Invention
The present invention relates to an OFDM transmitting and receiving apparatus for use in a radio communication system for transmitting an OFDM (Orthogonal Frequency Division Multiplexing) signal.
2. Description of the Related Art
In general, in radio communication systems, a signal is transmitted via multipath as a result of wave reflection due to, for example, buildings. Therefore, a received signal is distorted. The distortion varies depending upon place and time. In such radio propagation circumstances, the effective rate of data transmission can accordingly be maximized by changing the transmission conditions such as modulation form, coding rate, etc. Usually, a receiver accesses a known reference signal or data sequence to obtain a received-signal intensity or transmission channel response characteristics, and then the receiver sets transmission conditions suitable for the propagation circumstances.
If a to-be-transmitted signal is a wideband signal such as an OFDM signal, frequency selective fading occurs due to multipath. Upon the occurrence of frequency selective fading, the intensity will differ between OFDM signal subcarriers of different frequencies. Subcarriers of excellent receiving characteristics and subcarriers of degraded receiving characteristics will occur. Because of the existence of subcarriers of degraded received signal characteristics, it is very possible that errors may occur even if the average received signal intensity or transmission channel response characteristics of all subcarriers are high. Accordingly, even if transmission conditions suitable for the average propagation circumstances of the subcarriers are selected, the conditions may not significantly improve the effective data transmission rate.
As described above, in the conventional OFDM transmitting and receiving apparatus, transmission conditions are set in accordance with the average signal intensity or transmission channel response characteristics of all subcarriers. Therefore, if subcarriers of degraded received signal characteristics exist, the effective data transmission rate cannot be enhanced under the set transmission conditions.